In recent years there has been growing interest in the use of conjugated, semiconducting polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies up to 8%.
The conjugated polymer serves as the main absorber of the solar energy, therefore a low band gap is a basic requirement of the ideal polymer design to absorb the maximum of the solar spectrum. A commonly used strategy to provide conjugated polymers with narrow band gap is to utilize alternating copolymers consisting of both electron rich donor units and electron deficient acceptor units within the polymer backbone.
However, the conjugated polymers that have been suggested in prior art for use ion OPV devices do still suffer from certain drawbacks. For example many polymers suffer from limited solubility in commonly used organic solvents, which can inhibit their suitability for device manufacturing methods based on solution processing, or suffer from open circuit potentials (Voc) in OPV bulk-hetero-junction devices, or have only limited charge carrier mobility, or are difficult to synthesize by methods which are unsuitable for mass production.
Therefore, there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, and do not suffer from open circuit potentials (Voc) in OPV bulk-hetero-junction devices, or do so to a lower extent than polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing conjugated polymers containing thieno[3,2-b]thiophene-2,5-dione-3,6-diyl and/or furo[3,2-b]furan-2,5-dione-3,6-diylrepeating units and their thioketone derivatives (the numbers in the first formula indicate the positions on the thienothiophene or furofuran core):

It was found that conjugated polymers based on these units show good processability and high solubility in organic solvents, and are thus especially suitable for large scale production using solution processing methods. At the same time, they show a low bandgap, high charge carrier mobility and high oxidative stability and are promising materials for organic electronic OE devices, especially for OPV devices.
Monomeric derivatives of thieno[3,2-b]thiophene-2,5-dione have been studied in redox systems (Guenther, Erhard; Huenig, Siegfried. Chemische Berichte 1992, 125, 1235-41), demonstrating the electron deficiency of such a core (i.e. electron accepting unit). Other monomeric derivatives of thieno[3,2-b]thiophene-2,5-dione have also been used as electrophotographic photoreceptors (Hayata, Hirofumi; Hirano, Akira; Hirose, Hisahiro. Jpn. Kokai Tokkyo Koho 1992, JP04338761 A 19921126.), demonstrating photosensitivity and charge transport properties. However, it has hitherto not been suggested to use such compounds as recurring units in semiconducting polymers, especially for use in OFET or OPV devices.